Re: [PATCH v2 1/7] Compiler Attributes: Add __alloc_size() for better bounds checking

Kees Cook
2021-08-19T07:06:18Z

On Wed, Aug 18, 2021 at 04:19:12PM -0700, Andrew Morton wrote:
> On Wed, 18 Aug 2021 14:40:15 -0700 Kees Cook <keescook@chromium.org> wrote:
>
> > GCC and Clang can use the "alloc_size" attribute to better inform the
> > results of __builtin_object_size() (for compile-time constant values).
> > Clang can additionally use alloc_size to inform the results of
> > __builtin_dynamic_object_size() (for run-time values).
> >
> > Because GCC sees the frequent use of struct_size() as an allocator size
> > argument, and notices it can return SIZE_MAX (the overflow indication),
> > it complains about these call sites may overflow (since SIZE_MAX is
> > greater than the default -Walloc-size-larger-than=PTRDIFF_MAX). This
> > isn't helpful since we already know a SIZE_MAX will be caught at run-time
> > (this was an intentional design). Instead, just disable this check as
> > it is both a false positive and redundant. (Clang does not have this
> > warning option.)
> >
> > ...
> >
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -1078,9 +1078,13 @@ KBUILD_CFLAGS += $(call cc-disable-warning, stringop-overflow)
> > # Another good warning that we'll want to enable eventually
> > KBUILD_CFLAGS += $(call cc-disable-warning, restrict)
> >
> > -# Enabled with W=2, disabled by default as noisy
> > ifdef CONFIG_CC_IS_GCC
> > +# Enabled with W=2, disabled by default as noisy
> > KBUILD_CFLAGS += -Wno-maybe-uninitialized
> > +
> > +# The allocators already balk at large sizes, so silence the compiler
> > +# warnings for bounds checks involving those possible values.
> > +KBUILD_CFLAGS += -Wno-alloc-size-larger-than
> > endif
> >
> > # disable invalid "can't wrap" optimizations for signed / pointers
>
> Makefile has changed. I did this:
>
> --- a/Makefile~compiler-attributes-add-__alloc_size-for-better-bounds-checking
> +++ a/Makefile
> @@ -1003,6 +1003,12 @@ KBUILD_CFLAGS += $(call cc-disable-warni
> # Enabled with W=2, disabled by default as noisy
> KBUILD_CFLAGS += $(call cc-disable-warning, maybe-uninitialized)
>
> +ifdef CONFIG_CC_IS_GCC
> +# The allocators already balk at large sizes, so silence the compiler
> +# warnings for bounds checks involving those possible values.
> +KBUILD_CFLAGS += -Wno-alloc-size-larger-than
> +endif
> +
> # disable invalid "can't wrap" optimizations for signed / pointers
> KBUILD_CFLAGS += -fno-strict-overflow

Oh, er, where did "Makefile: remove stale cc-option checks" go? Ah, I
see now:
https://lore.kernel.org/mm-commits/20210814215814.W_qqW%25akpm@linux-foundation.org/T/#u

Looks like I just happened to pick the wrong linux-next. ;)

Thanks for the fix-up!

--
Kees Cook